A couple of years ago I asked about varnishing over shellac on some
bathroom doors. The old thread is at http://tinyurl.com/5bl3rz.

At any rate, one fine day when the family was gone and I could open
the windows and doors, I got lazy and just gave the doors and cabinets
a quick scuff with some 320 sandpaper and slapped a fresh coat of
shellac over the damaged areas; a couple of coats over the worst
areas.

The results have been spectacular! You can't tell where the finish
had previously been dulled and rough. Now it's nice and shiny. It
was so easy (I love shellac) that I think I'll just do it every year
or so as a quick preventative.

Thanks again to the NG.